Mention the --explain flag whenever an error occurs

[dosisod]

This is now the default behaviour. It can be disabled via --quiet, or by setting quiet = true in the pyproject.toml file. Update README accordingly.

Example:

$ refurb test/data/err_100.py
test/data/err_100.py:5:9 [FURB100]: Use `Path(x).with_suffix(y)` instead of slice and concat
test/data/err_100.py:8:9 [FURB100]: Use `Path(x).with_suffix(y)` instead of slice and concat
test/data/err_100.py:13:5 [FURB123]: Use `x` instead of `str(x)`

Run `refurb --explain ERR` to further explain an error. Use `--quiet` to silence this message

With --quiet flag:

$ refurb test/data/err_100.py --quiet
test/data/err_100.py:5:9 [FURB100]: Use `Path(x).with_suffix(y)` instead of slice and concat
test/data/err_100.py:8:9 [FURB100]: Use `Path(x).with_suffix(y)` instead of slice and concat
test/data/err_100.py:13:5 [FURB123]: Use `x` instead of `str(x)`